1. Field of the Invention
The present invention relates to a HEMT (High Electron Mobility Transistor) employing a group-III nitride.
2. Description of the Background Art
Group-III nitride semiconductors including GaN, because of their large bandgap, high breakdown field strength and high melting point, have been expected as high-power, high-frequency and high-temperature semiconductor device materials alternative to GaAs-based materials. HEMTs (High Electron Mobility Transistors) and the like have been studied and developed as devices which take advantage of such physical properties. An example of the HEMTs having been studied and developed is a heterostructure HEMT including a substrate made of sapphire, SiC or the like, a GaN channel layer formed on the substrate, and a so-called electron supply layer of AlGaN or AlN formed on the channel layer.
In a HEMT as described above, a two-dimensional electron gas is formed at the surface of the channel layer by a piezoelectric effect such that an electric field extending from the surface to the substrate is formed and by a spontaneous polarization effect due to an (a-axis) lattice constant difference between the channel layer and the electron supply layer. The Al-richer the AlGaN layer is, the greater the lattice constant difference becomes, and the piezoelectric effect and the spontaneous polarization effect increase accordingly.
For increase in performance of the HEMT, there is a need to enhance electron mobility while maintaining a high carrier density (or sheet carrier density) at the surface of the channel layer. Attempts to satisfy the need by devising the structure of the HEMT device have been known. (See, for example, Japanese Patent Application Laid-Open No. 2004-22577; Smorchkova et al., “AlN/GaN and (Al,Ga)N/AlN/GaN two-dimensional electron gas structures grown by plasma-assisted molecular-beam epitaxy,” Journal of Applied Physics, Volume 90, Number 10, pp. 5196–5201; Smorchkova et al., “AlGaN/AlN/GaN High-Power Microwave HEMT,” IEEE Electron Device Letters, Vol. 22, No. 10, pp. 457–459.)
There are a variety of electron scattering mechanisms that influence the electron mobility for semiconductor. The electron scattering mechanisms are roughly classified into a mechanism based on lattice vibrations and a mechanism based on the randomness in arrangement of atoms. The former is caused by thermal energy, has a temperature dependence, and hardly contributes to the reduction in electron mobility at cryogenic temperatures. The latter, on the other hand, is caused by the presence of lattice defects and impurities, and differs from the former in that the influence of the randomness in arrangement of atoms still remains even at cryogenic temperatures. Thus, the enhancement in electron mobility at cryogenic temperatures means the reduction in the influence of the randomness in arrangement of atoms upon the electron mobility.
Japanese Patent Application Laid-Open No. 2004-22577 discloses a technique of providing a highly crystalline underlying layer containing AlN at an interface between a substrate and a GaN layer in a HEMT having an Al0.26Ga0.74N/GaN heterostructure to improve the crystallinity of a channel layer and an electron supply layer which are formed on the underlying layer, thereby increasing the performance of the HEMT. This technique achieves the HEMT having a sheet carrier density of 1×1013/cm2 or higher and an electron mobility of 8000 cm2/V·s or higher at a temperature of 15 K.
The first article by Smorchkova et al., on the other hand, discloses a HEMT having an AlxGa1-xN/AlN/GaN structure in which an AlN layer is inserted at the interface of an AlGaN/GaN heterostructure for the purpose of avoiding alloy disorder scattering which is one of the scattering mechanisms resulting from the randomness in arrangement of atoms. This HEMT has a sheet carrier density of about 1.4×1013/cm2 or higher and an electron mobility of 4000 cm2/V·s at a temperature of 17 K when x ranges from 0.25 to 0.45. Also, a HEMT having an AlxGa1-xN/GaN structure in which AlN is not inserted is achieved. This HEMT has an electron mobility exceeding 15000 cm2/V·s when x is less than 0.2, in which case, however, the sheet carrier density is less than 5×1012/cm2.